The protagonist is the character whose desires and actions drive the screenplay. An effective protagonist has the following characteristics:
The ability to produce audience empathy is the most important quality of the protagonist.
Empathy should not be mistaken for sympathy. Sympathy is compassion. It is that feeling that makes people feel sorry for you. However, empathy is much more powerful. It is being able to fully understand why someone takes an action, whatever that action may be. Empathy makes an audience say, “If I was in the same situation, environment, or conflict, I’d do the same thing.”
The main character need not be likable or heroic, but the audience must be able to relate to him on some level.
At the beginning of the story, be consistent with the protagonist’s personality so the audience can get a handle on who he is and then gradually change him along the way. This will result in his character arc.
A formidable antagonist should cause a problem that will force the protagonist to draw on skills, strength, and resolve he did not know he had. This antagonist is usually external (situational, another person, social, institution, weather, etc.) or internal (shyness, extreme anger, bigotry, mental illness, etc.).
The primary problem, caused by the antagonist, should come with serious stakes and detrimental consequences if not dealt with successfully.
The audience must see the desire that is fueled by the problem. Even an internal desire must be expressed externally. In screenwriting, you have to show, not tell.
The protagonist must actively pursue his goal.
He should encounter obstacles that become progressively more difficult, forcing him to put everything on the line to achieve this goal. If not, the audience will wonder why the “hero” is inactive and will lose interest in the story.
The audience should have a sense of what the protagonist will reckon with at the climax.
After the final battle or climax in the script, the protagonist will have done one of three things: obtained his desire, failed to obtain his desire, or decided that he did not want the desire because it was no longer appropriate for him.
So always remember, the protagonist is the sun in the universe of your story. Craft him properly and he will light up the screen.